Fix integer bug in bias_variance_decomp

[rasbt]

Description

Fixes a bug in bias_variance_decomp where when the mse loss was used, downcasting to integers caused imprecise results for small numbers.

Related issues or pull requests

Fixes #743

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• Ran PYTHONPATH='.' pytest ./mlxtend -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., PYTHONPATH='.' pytest ./mlxtend/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./mlxtend

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Coverage increased (+0.004%) to 90.66% when pulling abac8cf on bias-var-decomp-int into aacee58 on master.